[mlir][EmitC] Remove the type from the OpaqueAttr

This removes the type from EmitC's opaque attribute. The value provided
as a StringRefParameter can always be emitted as is. In consquence the
constant and variable ops explicitly need to opaque attributes which are
no longer typed attributes.

Co-authored-by: Simon Camphausen <simon.camphausen@iml.fraunhofer.de>

Reviewed By: Mogball, jpienaar

Differential Revision: https://reviews.llvm.org/D131666